FACTORS SHAPING WINTER SOCIAL-ORGANIZATION IN HAZEL GROUSE BONASA-BONASIA - A COMPARATIVE-STUDY IN THE EASTERN AND WESTERN PALEARCTIC

Flock size, food abundance and dispersion patterns, and cover at forag ing sites were determined in winter for Hazel Grouse Bonasa bonasia at three study areas near the eastern edge of the species' range in east ern Siberia and one near the western edge in Sweden. Winter group size varied by habitat type; least in dense spruce forests in Sweden, when Hazel Grouse defended winter territories; intermediate in larch-birch ecotonal forests in Siberia; and greatest in open deciduous riparian forests in Siberia, where they formed cohesive, sexually mixed flocks. Group size in winter increased with increasingly open forests that ha d less cover at arboreal foraging sites but more winter food available , suggesting that the previously reported geographical differences in winter group size actually reflect habitat differences. A literature r eview supported this view. Winter food trees were significantly more a bundant and more evenly distributed in the Siberian habitats than in t he Swedish habitat. We suggest that groups were larger in more open ha bitats as a response to higher predation risk, but that food abundance also influences social organization. For example, abundant food may b e a prerequisite for flocking behavior to originate. We present a conc eptual model that can be used to test our hypothesis.